1. Field of the Invention
The invention relates to hybrid electric vehicles and more particularly to a strategy for requesting an engine ON or OFF state based on the condition of the vehicle battery.
2. Background Art
The need to reduce fossil fuel consumption and pollutants from automobiles and other vehicles powered by internal combustion engines (ICEs) is well known. Vehicles powered by electric motors have attempted to address these needs. However, electric vehicles have limited range and limited power coupled with the substantial time needed to recharge their batteries. An alternative solution is to combine both an ICE and an electric traction motor into one vehicle. Such vehicles are typically called hybrid electric vehicles (HEV's).
The HEV has been described in a variety of configurations. Some HEV systems require the operator to select between electric and internal combustion engine operation. In other configurations the electric motor drives one set of wheels and the ICE drives a different set. Other, more useful, configurations have developed. A series hybrid electric vehicle (SHEV) is a vehicle with an engine (most typically an ICE), which powers a generator. The generator, in turn, provides electricity for a battery and motor coupled to the drive wheels of the vehicle. There is no mechanical connection between the engine and the drive wheels. A parallel hybrid electrical vehicle (PHEV) is a vehicle with an engine (most typically an ICE), battery, and electric motor combined to provide torque to power the wheels of the vehicle. A parallel/series hybrid electric vehicle (PSHEV) has characteristics of both the PHEV and the SHEV. The PSHEV is also known as a torque (or power) splitting powertrain configuration. Here, the torque output of the engine is given in part to the drive wheels and in part to an electrical generator. The generator powers a battery and motor that also provide torque output. In this configuration, torque output can come from either source or both simultaneously. The vehicle braking system can also deliver torque to drive the generator to charge the battery (regenerative braking).
A desirable result of combining the ICE with an electric motor is that the ICE's fuel consumption and pollutants are reduced with no appreciable loss of performance or vehicle range. One of the benefits of the parallel HEV configurations is that the engine can be turned off during periods of low or no power demand from the driver (e.g., waiting for a traffic light) which improves fuel economy by eliminating wasted fuel used during idle conditions. The motor can then be used primarily to propel the vehicle under conditions of low power demand. In some configurations, the engine can be disconnected from the motor and powertrain when it is not running by opening a disconnect clutch. As power demand increases, the engine can be restarted and reconnected to provide the requested torque.
The two power sources in a PSHEV work together seamlessly with a common gear system to meet the driver demand for power without exceeding the power limits of the powertrain, including the limits of the battery subsystem. A vehicle system controller performs the coordination function in this split power powertrain. Under normal operating conditions, the vehicle system controller interprets the driver's demand for power as a function of acceleration or deceleration demand. It then determines when and how much torque each power source needs to provide to the transmission to meet the driver's power demand and to achieve specified vehicle performance (i.e., engine fuel economy, emission quality, driveability, etc.). The vehicle system controller determines the operating point of the engine torque and speed relationship.
In a hybrid electric vehicle with engine start/stop capability a battery controller may limit the maximum discharge power for various reasons. For example if the battery discharge limit becomes too low there may not be enough battery power available to start the engine. Also, when the vehicle is in reverse, the generator operates at a higher speed. Under such circumstances more battery power is required in order to start the engine.